1. Field of the Invention
This invention relates to a process for making dialkyl esters of a copolymer of a monoethylenically unsaturated dicarboxylic acid and a vinyl monomer.
2. Description of the Prior Art
FR-A-1,527,411 described copolymers of maleic acid anhydride, alkenes and alkyl vinyl ether monomers precipitation polymerization of the monomers, for example, in benzene in the presence of radical-forming polymerization initiators. The anhydride group of the copolymers can be hydrolyzed after copolymerization and also neutralized.
BE-A-710,985 described terpolymers made from alkyl vinyl ethers, maleic acid anhydride, and a third monomer, such as acrylic acid, methacrylic acid, methacrylamide, vinyl chloride, or styrene.
NE-A 68/10081 described a process of making copolymers of maleic acid anhydride, alkyl vinyl ethers or ethylene acetate, propylene acetate, or vinyl acetate, and 1-alkenes or long-chained vinyl alkyl ethers, in an inert solvent.
EP-A-0,428,956 described a process for making copolymers of maleic acid monoalkyl esters, alkyl vinyl ethers, and maleic acid anhydride. In this process, an excess of alkyl vinyl ether is used both as a solvent and precipitating agent. The specific viscosity or the chain length of the copolymers was controlled by the ratio of maleic acid monoalkyl ester to maleic acid anhydride.
U.S. Pat. No. 5,064,897 described the preparation of copolymers of monoalkyl maleates and alkyl vinyl ethers. In this patent, a 6- to 10-fold excess of alkyl vinyl ether was used, and the monomeric monoalkyl maleate was prepared prior to polymerization by the reaction of alcohols with maleic acid anhydride in equimolar ratios.
The conversion of polymers containing maleic acid anhydride units with alcohols in a polymer-analogous reaction is known, for example, from JP-A-1983/25,982, GB-A-1,233,468, DE-A-1,930,009, EP-A-0,310,079, and EP-A-0,461,489.
German Patent No. DE 4,216,318 described a one-pot method for the preparation of polymers containing, in polymerized from: (a) 2-50 mole % of alkyl vinyl ethers having 3.degree.-30.degree. C. atoms in the molecule; (b) 10-88 mole % of monoethylenically unsaturated dicarboxylic acid anhydrides; and (c) 10-88 mole % of dialkyl esters of monoethylenically unsaturated dicarboxylic acids, as characteristic monomer units, by polymerizing monomers (a), (b), and (c) at temperatures of 40.degree.-180.degree. C. in the presence of a radical-forming initiator. However, this one-pot process generally will produce inhomogeneous polymers because each of the three monomers have very different copolymerization reaction rates with the other monomers.
Accordingly, it is an object of this invention to provide a process for making dialkyl esters of a copolymer of a monoethylenically unsaturated dicarboxylic acid and a vinyl monomer, which does not require a one-pot polymerization of all monomers in the polymer.